[PATCH V1 0/2] Only allow SLAB_OBJ_EXT_IN_OBJ for unmergeable caches

[Harry Yoo <harry.yoo@oracle.com>]

While SLAB_OBJ_EXT_IN_OBJ allows to reduce memory overhead to account
slab objects, it prevents slab merging because merging can change
the metadata layout.

As pointed out Vlastimil Babka, disabling merging solely for this memory
optimization may not be a net win, because disabling slab merging tends
to increase overall memory usage.

So let's take a conservative approach and restrict SLAB_OBJ_EXT_IN_OBJ
to caches that are already unmergeable for other reasons.

Ideally this should be part of the slab/for-7.0/obj_metadata branch,
but due to a conflict with slab/for-7.0/sheaves, I based it on
slab/for-next (be36abd97e52).

Patch 1 factors out mergeability logic to slab_args_unmergeable().
This is used to determine mergeability before the cache is created.

Patch 2 uses the new function to allow SLAB_OBJ_EXT_IN_OBJ
only when merging is not allowed for other reasons.

Harry Yoo (2):
  mm/slab: factor out slab_args_unmergeable()
  mm/slab: only allow SLAB_OBJ_EXT_IN_OBJ for unmergeable caches

 mm/slab.h        |  1 +
 mm/slab_common.c | 27 +++++++++++++++++----------
 mm/slub.c        |  3 ++-
 3 files changed, 20 insertions(+), 11 deletions(-)

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2.43.0